Session I - Pelvic Trauma

*The Biomechanical Consequences of Perfect Secondary Congruence after Both Column Acetabular Fracture

Paul Tornetta III, MD, Boston University Medical Center, Boston, MA; Fred Behrens, MD; Richard Levine, MD; Regis Renard, BS, University of Medicine and Dentistry of New Jersey, Newark, NJ

Purpose: Clinical results after acetabular fractures are related to the congruence of the hip joint and the status of the roof (dome). Both column fractures allow for "secondary congruence" of the joint as both the anterior or posterior columns are separated form the intact ilium. The columns rotate away from each other allowing the head to remain relatively congruent with the articular segments. The long-term results after nonoperative management of both column (BC) fractures is better than other fracture patterns that affect the weightbearing surface of the acetabulum. The purpose of this study was to determine the changes in the loading pattern in a both column fracture model that simulates union with perfect secondary congruence.

Methods: Six cadaveric hemipelvic specimens and their respective femurs without preexisting disease or gross arthritic changes were used for the study. The specimens were potted via the sacroiliac joints and symphyseal bodies to allow for the natural deformation of the ilium under load. An MTS machine was used to load the specimens simulating single leg stance with a 25° medial inclination. Each specimen was preloaded at 500N for 5 minutes to reduce articular creep. Pressure sensitive film (Fuji) cut to a computer-generated design conforming to a sphere was applied to the femoral head and held in place with a latex prophylactic. Each intact specimen was then loaded to 1500N at a rate of 20 mm/min to obtain baseline pressure data with the rim and the acetabular margins used as reference points. A both column fracture (OTA type 62-C1.1) was then constructed by creating an intermediate anterior column fracture with a coping saw cut to within 4 mm of subchondral bone and then fracturing the articular surface by levering open the fracture with an osteotome. The anterior column fracture was then gapped using a 5mm shim at the level of the roof while perfect congruence with the femoral head was maintained. The fracture was then fixed in this position with standard reconstruction plates and position screws. The posterior column, which was fixed to the anterior column in the supra-acetabular region was then cut from the intact ilium through the superior notch and fixed to the ilium with plates. Thus, a model of perfect secondary congruence was created, as both the anterior and posterior joint surfaces were anatomically reduced to the femoral head but displaced 5mm from one another. This construct was tested in the same way as the intact specimen, using pressure sensitive film. The film was scanned to a computer for digital analysis using NIH Image 1.57 software. The acetabulum was divided into anterior, dome, and posterior thirds for analysis.

Results: With respect to the intact specimen, the contact area, mean pressure and peak pressure in the dome region all increased (p < 0.05) in the BC model. The contact area in the anterior articular region decreased (p < 0.05) as did the mean and peak pressure. In the posterior articular region the contact area and mean pressure decreased, but the peak pressure increased (not significant). These changes are summarized in table 1. Descriptively, the stress concentration shifted towards the fracture in all cases with the most anterior and most posterior articular regions having little contact in the fracture model. The highest pressures were seen in the dome region with the BC model manifesting 130 % of the mean pressure and 280 % of the peak pressure of the intact specimen.

Discussion: The reported results of nonoperative management of displaced both column acetabular fractures are better than those after nonoperative management of other displaced acetabular fractures affecting the weight bearing surface. This is thought to be related to the secondary congruence allowed for by displacement of the entire articular surface. The purpose of this study was to evaluate the best possible situation after BC fracture, that of perfect secondary congruence, where both the anterior and posterior articular surfaces are anatomically reduced to the femoral head. We found that even in this model, which has better congruence than the actual in vivo state, there was a significant increase in contact area and pressure in the region of the dome immediately adjacent to the fracture. Although the long-term effect of the changes in stress concentration seen in this model are not well defined, these findings represent the best possible situation after BC fracture and demonstrate significant abnormalities.

Conclusion: Stress concentration during simulated single-limb stance occurs adjacent to the fracture line in the dome of the acetabulum in a model of a both column acetabular fracture. Even union with perfect secondary congruence after a both column fracture may be associated with significant loading abnormalities that may lead to increased wear and arthritis over time.